Topic: Bleeding brakes

[ACL]

I had a brake failure today caused by a circlip on one of the brake pistons coming out of its slot. I can't imagine how it happened, but it occurred just as I was turning onto the runway for take off. Suddenly no left brake at all, so shut down and pushed the aircraft back to the apron. (Lucky it didn't happen on a crosswind landing, I suppose).

The circlip had partially pushed out of its slot and the pad had twisted slightly. Has anybody experienced or seen anything similar? I wondered if a small stone had got between the disk and pad causing an uneven force back against the brake pad. The brake fluid had leaked out of the caliper as a result, and emptied the reservoir, so once the circlip was replaced the system needed bleeding.

We tried pumping the lever with the bleed nipple unscrewed, but no fluid was forced through the system. Eventually resorded to pumping fluid into the bleed nipple and sort of 'reverse bleeding'. Result was operational but spongey LHS brake (RHS remained unaffected throughout). About 6 clicks now required for taxying, whereas before the brake just started to bite at full rudder travel with no brake... to respond to another thread.

Are we missing a trick on bleeding the brake? I'd be grateful for any hints and tips.

Many thanks,
James

[Dean Cross]

There is a school of thought that says the master cylinder is at an angle and the air gets trapped. Apparently tilting it seems to work but involves access under the belly and removal of a bolt. I have never personally tried it though.
Did you preesure bleed the system until the fluid rose bubble free in the reservoir?
DC

[Bob]

It was after various brake failures and ruining a set of tyres with weeping fluid we did the Cleaveland conversion!

http://www.dhchipmunk.com/Brakeconversion.htm

The Dunlop system can be a pain to bleed, at one point we put a long pipe on the bleed nipple and tied it to the engine cowl then left it to self bleed over night...
It sounds as though you still have a leak,maybe a damaged seal in the caliper?
good luck!
Bob

[ACL]

Another bleeding session and lots of fluid-soaked rags, and things seem to be back to normal. There must have been some air trapped which took a long time to work its way out.
Thanks,
James

[Dick Gower]

Another bleeding session and lots of fluid-soaked rags, and things seem to be back to normal. There must have been some air trapped which took a long time to work its way out.
Thanks,
James

They are a pain to bleed James;  I have tried bleeding from the wheel end and the reservoir;  even pressure bleeding with a fitting made up tp go into the reservoir and tried pressure bellding from both places.

Tim Becroft of Tocumwal Aviation  attaches the bleed to the wheel cylinders and uses vigorous rudder movements to move the bubbles downwards.

The trouble is that if you pause during the process the bubbles travel upwards again to the master cylinder and you are back where you started (same as when you let the fluid in the reservoir run out during the process!)

Tim also adds the reminder that  the master cylinder on the left is the one for the right wheel.

Regarding the circlip, I suspect it is was installed incorrectly.  Does the master cyl. push rod have two grooves in it from the circlip ears?  The Dunlop book (which is virtually unobtainable ) requires the ears on the circlip and hence the gap, to be positioned in line with the mounting lugs.  The gap is therefore downwards when the cylinder is installed so that it provides clearance for the push rod (which operates at a slight angle when extended).

The other thing that helps is to install the circlip with the sharp edge facing outwards.  If you look at it closely you will see that  it is smooth and slightly radiused on one side and sharp on the other.

Lastly make sure that the circlip groove is not full of debris which prevents it properly seating.

Don't forget to check the circlip orientation on the other side!

As for the leaking wheel cylinders, the best solution is to widen the groove for the seal and install an MS28775-216 o-ring instead of the original square-section AHO25019.  This is done under an Engineering Order and has been the norm out her in the colonies for many decades.  Some operators even double-groove the pistons and use two o-rings.  The same modification applies to the stationary seals on the outside of the wheel cylinders.

We also change the ACO4026 seal in the master cylinder to an MS28775-012 o-ring.

Hope that helps James.

[David H]

I just got a call from my mechanic.   He told me that one of my square section o-rings needs to be replaced.    I searched this site and found Dick's part numbers for the MS o-rings.    I hope that's the part he needs.   We'll know on Monday.    They had to bleed the brakes after they changed out the gear donuts.    I'm looking forward to flying it - well taxiing really - with the new rubber. 

Many thanks Dick!!!!

Dave

[David H]

I just wanted to post a follow up about my brakes.    We were able to use a mil-spec o-ring for the front seal on the left master cylinder.     Everything was working fine for a couple of months until we got some cold weather.   After the first chilly night the left wheel cylinder started leaking.    My mechanic replaced all four seals with the stock square shouldered o-rings.   The edges on the old seals were hard and clearly in need of replacement.   Everything checked out fine.   I made my run up and the brakes worked great so I departed.    On my first landing, I had no left brake and coasted off the right side of the runway onto the grass.   No matter what I tried I couldn't turn left.   I guess my brakes were set with a bit of drag - not a good thing.   My thinking is that when the left side started leaking there wasn't any drag on that side so off I went to the right.   I generally go very easy on the brakes and since this was my first landing after the brakes were rebuilt I made sure I was at a very slow pace before I used the brakes.     Since I was moving slowly, it wasn't a problem but I think it illustrates the problem with not being able to fully release the brakes.     I was very fortunate to miss the side lights.     After it came to rest, I tried full left rudder and a blast of throttle to try to get the plane to turn left but no luck so I shut it down and we pushed it back to the hanger.

We think that the reason the left wheel cylinder started leaking again after the seals were replaced had to do with the circle clip.   One of the ears was bent slightly and it was pushing just a bit on the upper piston.   It appears that this canted the piston just enough to lose the seal.    The problem with this theory is that the piston was working before I left.     Any other ideas?

So for now my brakes are working and seem to be adjusted perfectly.    I ran the engine up today to 1800 RPM and the brakes held just fine.    It really is an odd system.    I guess the designers felt the cable system was cheaper and easier than having two sets of master cylinders and the associated plumbing.     I use to have an old +4 Morgan.   It had the same type of brake lever that wouldm't set until you pushed button down.   I think they referred to it as a "fly off" brake.    The chipmunk needs it so either pilot can release the brakes.     I don't know why they put it on the Morgans.

I've read about the problems with bleeding the brakes.     The standard method was not very effective.    We had very good luck bleeding the brakes with a Mightvac brake bleeding kit.    It took a long time to get all the air bubbles out but it's easy and can be done by one person from the bleed nipple at the wheel cylinder.    We also tried pumping fluid back from the bleed nipple up into the reservoir but that method didn't work. 

[Dick Gower]

I sympathise with your brake problems David and have owned +4 Morgan for the last 35 years with the very knock-off hand brake you describe.

It sounds as though the circlip was not fully bottomed in the groove.  When fitting the circlips to the wheel cylinders it is better to install with the sharp edges facing outside.  (Circlips have a sharp side and a curved side).  Also it is important to thoroughly clean out the circlip groove before installing the circlip.  A match stick or wooden toothpick works well for this and does not remove metal.

Also don't forget to get the correct orientation for the circlip in the master cylinder: the open end must be aligned with the mounting lugs otherwise it wears the push rod.

Can you give some more details on the Mightvac system?

[David H]

Hi Dick,
Here's a link to the mighty-vac product
http://www.amazon.com/Mityvac-MV8000-Brake-Bleeding-Kit/dp/B00265M9SS

This is apparently a newer model than the one I have.   One problem with it is making sure the vacuum jar is sealed to the pump.    It seems that most reviewers either hated it or loved it.

For now, my brakes are working fine but I think I'm going to try to get machinest to create new pistons with groves for the standard o-rings this Spring.   

Thanks for the great tips!

Dave

[Dick Gower]

Yes, that is a good idea David.

There is room for two o-rings and it is worth adding the second one.

Dick

[proplover]

Seem to be 2 things re brakes going on here. ACL's was the circlip holding the fixed piston in on the brake caliper. We found that ours had a bent 'ear' on one. As the inner moving piston was weeping I decided to replace the circlip at the same time. The circlip purchased from Skytrade was a lot stronger than the 'original' and certainly went into the slot with a clunk! It has never moved since!
There is a method in bleeding the brakes which is a combination of the brake lever, rudder bar and bleed screw - it just doesnt seem to work as effectivily by leaving the rudder bar central. When sequenced correctly with either left or right rudder for the appropriate brake you should find that the presure generated will have the fluid hitting the underside of the wing!
Re the brake master cylinder bit, if in the UK be carefull if your using O rings that are not in the manual for Chipmunks even if they are Mil Standard - check with de Halivalland Support .
Its frustrating that so many usefull mods which are available to our overseas friends are just not allowed in the UK with no real reason other than 'there not in the (1950's) manuals so you cant do that\use that'

[Dick Gower]

Perhaps even three topics proplover.

(1)  The circlips do lose their tension in service and if they often need replacement for this reason.  If they are not removed and installed with proper circlip plyers there is a risk of permanently damaging them.

(2)  The brake bleeding method you describe is used by one of the most experienced Chipmunk maintainers here in Oz and yes, vigorous use of the rudder bar is definitely a necessary part of the process.

(3)  The o-ring substitution in the brake system is such an improvement over the original seals that it would make a lot of sense to get official approval by for use world wide.  O-rings can be used to great advantage in the master cylinder pistons, wheel cylinder pistons and wheel cylinder seals, a total of ten per aircraft.  I will see what might be done about that officially.

[john henderson]

I spoke to de Havilland about the Australian mod a couple of years ago.  They said that they do not have type approval for the Dunlop master cylinder as it is not a de Havilland product.  Therefore they cannot approve any mods!  They did say that if I could get the drawings from Dunlop, they would look into it.  Dunlop will have nothing to do with it (product liability!).  De Havilland also said that to replace the master cylinders with a different type would probably be a major mod with all of the associated costs.  It cannot be done because only Dunlop can authorise it!

So there you have it, add a second proper seal - it is an improvement, it would improve safety; it is a very minor engineering change- a second groove cut into the piston (which has ample meat on it), but it makes your aeroplane non airworthy. 

We are barking mad in this country!  why do we put up with it?

[Bob]

Seconded John

[proplover]

Thirded John